1. Field of the Invention
The invention relates to fluorescent lamps, and is directed more particularly to improvements in specialty lamps, such as small diameter low power fluorescent lamps and to methods for making electrode assemblies for such lamps.
2. Description of the Prior Art
It is known to provide a fluorescent lamp with a glass tubular body defining a discharge space, and a pair of electrode assemblies disposed in the discharge space in opposed relation to each other. Each of the electrode assemblies includes an arc discharge electrode and a glow discharge electrode disposed adjacent to each other. An electron-emitting substance is incorporated in the arc discharge electrode and is, in operation, vaporized and emitted from the arc discharge electrode and captured by the glow discharge electrode.
It is further known to provide an arc discharge electrode which comprises a sintered body containing therein an electron radiating substance. Such is disclosed, for example, in U.S. Pat. No. 5,304,893, issued Apr. 19, 1994, to Y. Nieda.
Many current small diameter fluorescent lamps are of the type described above and are provided with electrode assemblies as described above. Such lamps require either a high operating voltage or, in some cases, separate power to heat the electrodes. There is a need for a small diameter fluorescent lamp in which the electrodes operate thermionically, at low voltage and without need of external heater power. There is an attendant need for a method for making electrode assemblies for such lamps.
Current cold cathode, small diameter (less than 6 mm inside diameter) and low pressure (less than 100 torr) lamps exhibit limited life because of changes in lamp color, rapidly followed by cracking of the lamp envelope proximate to the electrodes. It has been found that lamp color changes are caused by xe2x80x9cgas trappingxe2x80x9d. That is, gas ions which drift near the glow discharge electrodes are accelerated in large glow discharge electrode fields and slam into the glow discharge electrode surface, sometimes leaving gas particles trapped below the surface of the glow discharge electrode. A reduction in gas atoms in the lamp shifts the discharge electron energy distribution to higher energies. Higher energy electrons excite higher energy levels within the gas atoms, causing a change in the emission spectrum, that is, a color shift. Sputtering, which necessarily accompanies gas trapping, knocks metal atoms from the electrode and sputter remnants drift to, and deposit on, the inside of the lamp glass envelope. The discharge attaches to the metallic coating, creating large heat flux to the glass surface. Cooling in the glow discharge electrode region causes mechanical stresses in the lamp glass envelope resulting from the differences in thermal expansion properties between the glass and the sputtered metal. This differential thermal expansion causes the lamp envelope to crack.
There is therefore a need for a small diameter low pressure lamp in which the electrode assemblies are not subject to gas trapping and which exhibit a substantially longer life than current standard electrodes. There is further a need for a method for making electrode assemblies for such lamps.
An object of the invention is, therefore, to provide a small diameter low pressure fluorescent lamp having electrode assemblies which operate at low voltage and without the need of external heater power.
A further object of the invention is to provide a method for making electrode assemblies for such a small diameter low pressure lamp.
A still further object of the invention is to provide a small diameter low pressure fluorescent lamp having electrode assemblies which are not subject to gas trapping, permitting the lamp to exhibit a longer working life.
A still further object of the invention is to provide a method for making electrode assemblies for such a small diameter low pressure lamp.
With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a fluorescent lamp comprising a glass tubular body defining a discharge space, first and second electrode assemblies mounted in the discharge space in opposition to each other, each of the electrode assemblies comprising a first electrode and a second electrode. Each of the first electrodes comprises a metal lead wire with an electron-emitting material disposed on a free end thereof. Each of the second electrodes comprises a cup-shaped body coaxially surrounding one of the first electrodes and the electron-emitting material disposed on the first electrode, the second electrode cup-shaped body and the electron emitting material therein forming an annular gap therebetween.
In accordance with a further feature of the invention, there is provided a method for making an electrode assembly for small diameter low pressure fluorescent lamps, the method comprising the steps of providing a metal lead wire having a free end, dipping the wire free end into liquid solvent in which an emitter material is disposed, crimping the wire in a metal tube with the wire free end and emitter material thereon recessed inside the tube, vacuum baking the tube, wire and emitter on the wire, and sealing the wire in a glass tubular body portion of the fluorescent lamp.
In accordance with a still further feature of the invention, there is provided a method for making an electrode assembly for small diameter low pressure fluorescent lamps, the method comprising the steps of providing a metal lead wire having a free end, sealing the lead wire in a high temperature glass electrode, the electrode comprising a cup-shaped body, with the lead wire disposed substantially centrally, widthwise, of the cup-shaped body, and dipping the wire free end into a liquid solvent in which an emitter material is dispersed.
The above and other features of the invention, including various novel details of construction and combinations of parts and method steps, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices and methods embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.